1. Field of the Invention
The present invention concerns a powder mixture for powder metallurgy and a binder used therefor. A starting powder for powder metallurgy comprising a metal powder such as an iron powder or a steel powder as a basic component and, blended therewith, a powder of physical property improving ingredients such as alloying elements and graphite and a lubricant powder is incorporated with a copolymer ingredient of a predetermined composition ratio, so that segregation of the powder of physical property improving ingredient and the lubricant powder is suppressed without deteriorating the physical property of the metal powder as the base component and dusting of the powder upon handling is suppressed.
2. Description of the Prior Art
In powder metallurgy using a metal powder such as an iron powder or a steel powder as the main starting material, alloying elements such as copper, nickel, chromium and molybdenum, a powder of physical property improving ingredients such as graphite, phosphorus and sulfur and a lubricant powder such as zinc stearate are sometimes blended in order to improve the physical property (such as strength characteristics or workability) of sintering products. Usually, particle size, specific gravity, etc. of the powder of physical property improving ingredients and the lubricant powder vary considerably. For instance, since each specific gravity differs in a case where the base metal powder is an iron powder or a steel powder (hereinafter collectively referred to as an iron-steel powder, and the powder of physical property improving ingredients is graphite, phosphorous or the like, the difference of the specific gravity tends to cause segregation in the course of handling after mixing up to molding to worsen the characteristics and the homogeneity of sintering products. Further, if the lubricant powder used for extending the die life causes segregation, it sometimes results in increase of the drawing pressure upon taking out a molding product from the die or causes fluctuation in the powder characteristics.
As a means for preventing such segregation, methods of depositing, for example, a graphite powder to an iron/steel powder or the like by using a organic binder have been proposed as disclosed in Japanese Patent Laid Open Sho 56-136901 and Sho 63-103001.
However, since the organic binders as disclosed in the above-mentioned literatures are hydrophilic, they involve a problem of absorbing moisture to lower the flowability during storage or promoting rusting of the base metal powder, which may rather worsen the quality of powder metallurgy products. Further, since the organic binder has a greater effect of increasing the bonding strength between the iron/steel powders each other rather than increasing the bonding strength between the iron/steel powder and the powder of physical property improving ingredients or the lubricant powder, the effect of preventing segregation of graphite or the like is insufficient and a great amount of the binder has to be blended in order to obtain the more excellent effect. As a result, since the bonding (agglomeration) between each of the iron/steel powders become remarkable, steps of repulverization or sieving after mixing and drying are indispensable.